1. Field of the Invention
This invention relates to a seat belt retractor which tightens a seat belt when an automobile body undergoes acceleration or deceleration exceeding a predetermined value, so that the seat belt secures a passenger to a seat.
2. Description of the Prior Art
A previously known seat belt retractor comprises a reel rotatably supported in a housing and winding a seat belt. A winding spring urges the reel in the direction in which the seat belt is wound. A tightening spring is directly connected to the winding spring. A lock mechanism is provided between the tightening spring and the winding spring so as to stop transmission of the resilient force of the tightening spring to the winding spring.
In this known seat belt retractor, in a normal condition, only the resilient force of the winding spring is therefore transmitted to the reel. On the contrary, when the automobile body receives shock, "shock" being defined herein as acceleration or deceleration of the automobile exceeding a predetermined value, the lock mechanism releases the tightening spring so that its resilient force is also transmitted to the reel. That is, only the winding spring urges the seat belt to the winding direction in a normal condition, but the tightening spring urges the winding spring so that both springs wind seat belt when shock is applied to the automobile body.
It takes time, however, for this known retractor to wind the seat belt as the winding spring first is wound by the tightening spring, and then the reel is rotated to wind the seat belt. Also, it is necessary for the tightening spring to have a large torque to tighten the winding spring and the reel. Naturally, this means a large tightening spring, which in turn enlarges the retractor.